The explosive growth in the volume and variety of multi-media telecommunication applications continues to drive speed demands for internet traffic and motivate research in backbone fiber-optic communication links. Coherent communications and electronic digital signal processing (DSP)-based receivers have been accepted in recent years as the next-generation standards for long-haul systems due to their flexibility, scalability and ability to compensate for various transmission impairments, including fiber nonlinearity. As fiber nonlinearity places a limit on achievable spectral efficiency, large effective-area (Aeff) single-mode fibers (SMFs) have been designed for reducing nonlinearity penalties.
However, the spectral efficiency of an optical fiber increases slowly with increasing effective area, so another solution is needed to increase system capacity. Recent experiments have demonstrated that it is possible to transmit signals in more than one spatial propagation mode of a few mode fiber (FMF) using multiple-input multiple-output (MIMO) techniques. Few mode fibers are particularly attractive for this application because the computational complexity directly scales with number of modes, and utilizing only a few modes reduces the risk of modal mixing that can lead to bit error rate penalties from multipath interference (MPI).
Few mode optical fibers previously proposed for optical fiber communications systems have either step index or parabolic cores in which the core diameter is increased relative to single mode fiber to support at least the LP11 mode in addition to the fundamental LP01 mode. With both of these core designs, there are large delay differences between the fundamental LP01 mode and the LP11 mode at one or more wavelengths in the 1550 nm window. These large delay differences make it difficult to demultiplex the optical signals in the time domain using MIMO.
Accordingly, a need exists for alternative designs for few mode optical fibers with low loss and small differential group delays (DGD).